A first known category of photovoltaic cells converts light into electricity by exploiting the photovoltaic effect that appears at the junction of semi-conductor materials. The semi-conductor material fulfils both the function of light absorption and of separation of the resulting electrical charges (electrons and holes). The material has to be of great purity, free of any defects, otherwise electrons and holes recombine before being able to be separated.
The present invention concerns a second type of photovoltaic cells called electrochemical cells, which include a semi-conductor material that is normally insensitive to visible light because of its forbidden bandwidth, and that only starts to absorb in the near ultraviolet. Such a material can nonetheless be sensitized by adsorption of a colouring agent such as a transition metal complex which allows a conversion rate between an incident photon and an electron close to one. After having been excited by the absorption of a photon, the colouring agent can transfer an electron into the conduction band of a semi-conductor material. The electric field prevailing at the core of the semi-conductor material enables the electron to be extracted. After transferring the electron, the colouring agent returns to the fundamental oxidized state. Recombination between the electron in the semi-conductor material conduction band and the hole on the oxidized colouring agent is much slower than the reduction of the oxidized colouring agent by a mediator. Consequently, the charge separation is efficient.
The cells of the type described hereinbefore generally include a first transparent front substrate and a second back substrate, which generally can either be transparent or not. These two substrates each include on their faces that are opposite each other a first electrode also called the counter-electrode, and a second electrode usually called known as the photo-electrode. These electrodes are to be connected to an electrical supply circuit and are conventionally made in the form of a thin layer of a transparent conductive oxide such as a mixture of indium/tin oxide or antimony/tin oxide.
The two substrates are joined to each other by a sealing frame, which extends along the periphery of the latter. This sealing frame defines a sealed volume for retaining the semi-conductor material deposited in layers on one of the substrates and an electrolyte containing the aforementioned mediator.
The present invention also concerns so-called electro-optical cells, in particular liquid crystal cells, which, like electrochemical photovoltaic cells, include:                a first transparent front substrate whose top surface forms the front face of the cell;        a second back substrate which can also be transparent or not transparent, whose bottom surface forms the back face of said cell;        the substrates each including on their faces that are opposite each other at least one electrode, these electrodes being intended to be connected to a display control circuit, which, by applying appropriate electrical voltages to selected electrodes, is able to alter the transmission or reflection features of an optically active medium;        the substrates being joined by a sealing frame defining a sealed volume for retaining an optically active medium, and        connection means for setting up the electrical connection between each electrode and the display control circuit.        
The sealing frame assures the hermeticity of the edges of the cell, thereby efficiently retaining the active medium contained by the cells, and protecting this medium from gas diffusion phenomenon from the surrounding atmosphere which could compromise the perenity of the cells.
Usually, deposition of the sealing frame is carried out by screen printing, a method whose implementation can irremediably damage all of the fragile structures such as the electrical connections or spacers which have already been deposited when the screen printing step occurs. In fact, the screen printing technique, which consists, let us recall, in depositing a material of paste-like consistency through the unobstructed mesh of a screen, for example made of nylon or stainless steel, with a very fine mesh using a squeegee that is actuated by hand or mechanically, is a technique whose implementation generates not insignificant mechanical stress which is often harmful to the already deposited neighbouring structures, such as the alignment layers, the spacing bars or the electrical connections.
As is known, the spacers are supposed to maintain a constant space between the two substrates of the cells and give the latter satisfactory mechanical rigidity. In known methods to date, the distance between the two substrates is generally maintained by balls or discontinued fibres of perfectly controlled geometrical dimensions, spread over one of the substrates before the second substrate is set in place. This initial technique has certain drawbacks, such as, in particular, a high price and random positioning of the balls that can form optical diffusion centres, at the core of the pixels of a matrix display, adversely affecting the appearance of the display. It has thus been proposed to replace such balls with continuous spacers obtained by depositing a layer of photoresist material of the desired thickness over one of the substrates, this layer then being structured to give it the shape of the desired spacers. This latter technique is the only one capable of guaranteeing the mechanical rigidity necessary for certain types of liquid crystals. It further prevents the aforementioned phenomenon of optical diffusion, which damage the appearance of the spacers insofar as the spacers can be deposited selectively at locations chosen in advance, particularly outside the pixels. Finally, spacers structured by photolithography can have an adhesive power enabling the two substrates to be assembled to each other.
As will have been understood from the foregoing, the spacers play a particularly decisive part in the proper working of the cells with an optically or electro-optically active medium with which the present invention is concerned. Unfortunately, these spacers, when they are made of photoresist, can be irremediably damaged during screen printing deposition of the sealing frame.
Another drawback which weighs on the screen printing deposition techniques for sealing frames lies in the fact that it is difficult to control with precision the final dimensions of the frames. In fact, when the top substrate is applied onto the bottom substrate, the sealing material is compressed and tends to spread out via the effect of the pressure exerted, such that the width of the sealing frame can only be controlled with a precision typically of the order of a tenth of a millimeter. Moreover, the inner barrier of sealing frames deposited by screen printing, which is in contact with the liquid crystal, usually has irregularities in shape, such that these frames have to be arranged at a sufficient distance from the electrodes to prevent them from overlapping therewith. Such a situation may be acceptable if there is a large enough space between the edges of the cell from where the connections emerge and the actual active zone of the cell. However, as soon as one tries to reduce the dimensions of the dead zone reserved for the connector technology, in order to optimise the surface of the display zone of the cell or to answer problems of bulkiness, the precision offered by screen printing techniques is no longer sufficient.
The present invention concerns, finally, Microsystems of the fluidic type such as pressure sensors, micropumps or even micro-mixers for channelling the flow of liquids in a controlled manner. These micro-structures have, in fact, a large number of analogies with liquid crystal display cells and electrochemical photovoltaic cells. They include, in particular, two substrates separated from each other by a constant distance and between which channels are arranged. These channels, inside which the liquid or gaseous fluids flow, are conventionally etched in the volume of said substrates. During assembly of the substrates, which may occur, for example, by anodic welding, the nature of the materials used to make said substrates often imposes large constraints, in particular in terms of the welding temperature and the electric voltages applied. These constraints can be harmful to the structures already deposited on the substrates prior to welding.
It is an object of the present invention to overcome the aforementioned problems in addition to others still, by providing a manufacturing method, particularly for a liquid crystal display cell, which is easy to implement and which limits especially the risk of damaging elements already deposited in the cell.
The present invention also concerns a device with an optically or chemically active medium for implementing the method according to the invention.
Thus, according to its first aspect, the present invention concerns a method of manufacturing at least one device defining a volume for retaining a fluid or a sensitive material capable of changing physical properties, particularly optical properties, via application of a voltage, or electrical properties via stress or radiation, this device including at least a first front substrate and at least a second back substrate held at a constant distance from each other, these two substrates being joined by a sealing joint, which defines the volume for retaining the sensitive medium or fluid,
this method being characterised in that it includes the steps of:                structuring at least one wall, which defines via its inner lateral face the volume for retaining the sensitive medium or fluid, on one of the substrates;        joining the second substrate to the first substrate;        introducing a sealing material capable of flowing into the gap defined by the external lateral face of the wall and the two superposed substrates until at least a part of the volume of said gap is occupied by the sealing material; and        solidifying the sealing material so that the latter forms the sealing joint.        
Owing to these features, the present invention provides a method of manufacturing devices such as, particularly, liquid crystal display devices, which enables previously deposited fragile structures such as connection elements, to be embedded in the sealing material, thus greatly limiting the risk of damaging such structures. Such a result would also be difficult to obtain by screen printing without any risk of ending up with an excess thickness, locally, which would inevitably lead to imprecise spacing between the two substrates during the subsequent assembly thereof.
According to a first implementation variant of the method of the invention, a layer of photoresist material is deposited on one of the substrates, said photoresist material will then be structured by photo-etching techniques in order to give it the shape of one or several walls.